Gruul Gatemaster

When Gruul Gatemaster enters the battlefield,
you get {e}{e}{e}{e}.
Whenever a Gate card enters the battlefield under
your control, search your library for an centaur
card, put it into your hand, then shuffle your
library, if you got an centaur card this way, you can cast it for {e}{e} instead of it's normal casting cost.
{r}{g}, {t}: search your library for a Gate card, put it
onto the battlefield tapped then shuffle your
library.